1. Field of the Invention
The present invention relates to a flexible printed circuit board, and to an optical transmitter-receiver module and an optical transmitter-receiver which are each equipped with such a flexible printed circuit board.
2. Description of Related Art
For electrically connecting a flexible printed circuit board with other types of board or the like, a method has been used to connect them by means of a connector for connecting the flexible printed circuit board. Further, for transmitting high-frequency signals over a printed circuit board, a printed circuit board with a microstrip line structure has been employed.
FIGS. 1-4 are explanatory illustrations of a structure of a flexible printed circuit board 50 as related art, which is electrically connected to a connector. FIG. 1 is a plan view of the flexible printed circuit board 50 for showing an outline thereof, in which parts of its configuration is, for purposes of clarification, indicated in a transparent state by a broken line. FIG. 2 is a sectional view of the flexible printed circuit board 50 taken along lines M-M of FIG. 1 for illustrating an outline thereof. FIG. 3 is a plan view of a first wiring layer 3, described later, of the flexible printed circuit board 50, as viewed from an upper side in FIG. 2. FIG. 4 is a plan view of a second wiring layer 5, described later, of the flexible printed circuit board 50, as viewed from a lower side in FIG. 2. Further, FIG. 5 is a cross-sectional view of the flexible printed circuit board 50, a printed circuit board 18, and a flexible printed circuit (FPC) connector 7 mounted on the printed circuit board 18 for illustrating a condition where the flexible printed circuit board 50 is connected to the FPC connector 7. It is to be noted that the flexible printed circuit board 50 is illustrated as one shown in FIG. 5 with it being taken along the lines M-M of FIG. 1 and the printed circuit board 18 is shown only partially.
As shown in FIGS. 1-4, the flexible printed circuit board 50 is formed by alternately laminating vertically the first, second and third insulation layers 2, 4, 6 and the first and second wiring layers 3, 5. The first insulation layer 2 and the third insulation layer 6 are made of, for example, a cover lay as a protection film. In a predetermined region of an end of the flexible printed circuit board 50, for purposes of making electrical connection with the FPC connector 7, the third insulation layer 6 is in an unformed state. On an upper surface of the first insulation layer 2, a cover plate 53 is provided to prevent any damage from occurring at times of making connection with the FPC connector 7.
As shown in FIGS. 1 and 3, on the first wiring layer 3, a solid pattern ground layer 12b is formed. As shown in FIGS. 1 and 4, signal connection pads 52 for connection of a signal line 9, which will be described later, are equipped at a portion of the second wiring layer 5 that is in the vicinity of the end of the board, as connection pads for electrical connection with the FPC connector 7. Ground connection pads 51 for connection of the ground layer 12b are also equipped at a portion of the second wiring layer 5 that is in the vicinity of the end of the flexible printed circuit board 50, as connection pads for electrical connection with the FPC connector 7. Each of the ground connection pads 51 is connected to the ground layer 12b that is formed on the first wiring layer 3, via a ground via hole 54 passing through the flexible printed circuit board 50. Further, on the second wiring layer 5, a pair of signal lines 9c and 9d, which are microstrip lines, are wired in such a way that they can be respectively connected to the signal connection pad 52.
Further, as shown in FIG. 5, the FPC connector 7 is mounted on the printed circuit board 18, which includes a signal wiring layer 18a, an insulation layer 18b, and a ground layer 18c, and is connected to the flexible printed circuit board 50. The FPC connector 7 includes a resin-made housing 36 and a predetermined number of metal-made contacts 35 within the housing 36, each contact being constituted of an FPC connection section 35a that comes into contact with the connection pads of the flexible printed circuit board 50, a support section 35c for supporting the housing 36, and a lead section 35b to be connected to the printed circuit board 18. The metal-made contacts 35 are arranged in parallel to each other with predetermined spacing therebetween. Spacing between the connection pads of the flexible printed circuit board 50 correspond to spacing between the contacts 35.
The contacts 35 of the FPC connector 7 are respectively soldered to connection pads, not shown, which are formed on the outermost signal wiring layer 18a of the printed circuit board 18 at a position of the lead section 35b of each of the contacts 35, which is indicated by P. The connection pads of the printed circuit board 18 are respectively connected to a signal line or a ground pattern, neither of which is shown. The signal line or the ground pattern is formed on the signal wiring layer 18a of the printed circuit board 18.
The FPC connector 7 shown in FIG. 5 is of a so-called lower-contact type, so that the flexible printed circuit board 50 is thus connected to the FPC connector 7 in a condition where the connection pads of the flexible printed circuit board 50 are positioned toward a lower surface thereof, as indicated by O.
In such a configuration, a high-frequency signal current flows through the signal lines 9 on the flexible printed circuit board 50, the contact 35 of the FPC connector 7 that corresponds to each of the signal lines 9, and the signal line formed on the printed circuit board 18. At this time, a feedback current for the signal current flows in an opposite direction thereto through the ground layer 12b on the flexible printed circuit board 50, through the ground via hole 54, through the contact 35 of the FPC connector 7 that corresponds to a ground line, and through the ground layer 18c on the printed circuit board 18. FIG. 6 is a plan view of the flexible printed circuit board 50 for showing the flows of the signal current and the feedback current therein, which are illustrated on the ground layer 12b provided on the first wiring layer 3 and the signal lines 9 provided on the second wiring layer 5 in the flexible printed circuit board 50. On the flexible printed circuit board 50, as indicated by arrows Q in FIG. 6, if signal currents flow through the signal lines 9, feedback currents flow through the ground layer 12b on the first wiring layer 3, as indicated by arrows R.
Alternatively, a connector for flexible printed circuit board connection has been proposed which facilitates the insertion of a flexible printed circuit board with lower degree of insertion force (see, Japanese Patent Application Publication No. 2002-50423).
The connector for flexible printed circuit board connection disclosed in the above Japanese Patent Application Publication is equipped with a board insertion section into which a flexible printed circuit board is inserted and, on the opposite side thereof, a cover insertion section into which a slide cover is inserted in such a manner that it can be retreated. Further, this connector is also equipped with a contact that applies pressure to, and releases pressure from, a flexible printed circuit board inserted into the board insertion section, respectively in response to the insertion of a slide cover and to the withdrawal thereof. With such a configuration, it becomes possible to insert a flexible printed circuit board with lower degree of insertion force in condition where it does not interfere with any operations of the slide cover.